1. The Field of the Invention
The present invention relates to the field of fuel cell apparatus, and in particular, to electrical connector apparatus for planar solid oxide fuel cell (SOFC) stacks, and in particular to current collector apparatus for removal of current from such SOFC stacks.
2. The Prior Art
The removal of current by typical prior art current collector apparatus in multicell SOFC stacks is accomplished by attaching current collectors to the end interconnects of the multicell on surfaces, at least some of which are conventionally exposed to air (or an oxidizing) atmosphere. Such prior art apparatus include U.S. Pat. No. 5,270,131 to Diethelm et al.
Current collection efficiency is dependent on the bulk electronic conductivity of the current collector as well as the interfacial contact resistance between the current collector and the stack end plates. The current collectors for typical planar SOFCs are typically made of precious metal mesh or foil, typically of platinum or silver-palladium alloy. The use of precious metals, for their oxidation resistance properties, has the potential drawback of being costly.
In order to avoid the cost of using precious metals, other oxidation resistant alloys have been considered. However, current collectors made of such alloys achieve oxidation resistance through the formation of an electrically insulating layer at the collector surface. Accordingly, the useful life of such collectors may be limited. In addition, advanced bonding processes are required to attach the current collectors to the stack end interconnects, in order to prevent or reduce the build up of an insulating oxide layer at the interfaces of the collectors to the interconnects.
Other prior art apparatus have been configured to go to the extreme of immersing the entire module or stack in a reducing atmosphere (such as fuel gas) except those portions which must be exposed to oxidizing atmosphere. Irino et at., U.S. Pat. No. 5,198,312 is an example of such a configuration, and even in Irino et al. '312, it is only the end collectors, and not the individual cell or plate collectors, which are so protected.
It would be desirable to provide an SOFC configuration, particularly for planar SOFCs, in which the current collectors for individual cells or plates are protected from oxidizing effects.
It would additionally be desirable to provide an SOFC configuration, particularly for planar SOFCs, and especially for multistack SOFC modules, in which series and parallel current collection paths are provided to provide robustness to the multistack module, wherein if one stack experiences current collection problems, other collection pathways are provided, so that the overall module can remain operational.